COMBINED STUDIES OF THE DIELECTRIC-PROPERTIES OF NATURAL STILBITE SINGLE-CRYSTALS

The dielectric properties of single crystals of natural stilbite were studied by means of the thermally stimulated depolarization currents (TSDCs) over the temperature range 77-300 K and dielectric relaxation spectroscopy (DRS) over the frequency range 5 x 10(-3) Hz to 65.5 kHz, under conditions of varying polarization, hydration and temperature. The TSDC spectra reveal the presence of three main bands: (i) a complex band with intensity maxima at temperatures varying from 120 to 140 K, and activation energies between 0.21 and 0.39 eV; (ii) a simple relaxation band with an intensity maximum around 210 K and an activation energy in the range 0.55 +/- 0.06 eV; and (iii) a complex band at around 228 K, with activation energies around 0.85 and 1.55 eV. The low temperature band is ascribed to the relaxation of bulk water, and the intermediate temperature band to a vacancy-jump relaxation mechanism involving Na+ ions and vacant cationic sites, The higher temperature band is discussed in terms of a Maxwell-Wagner interfacial polarization. Dielectric measurements have identified a large dielectric relaxation peak in the kHz frequency range and a second, much smaller dielectric loss centred at approximately 30 MHz. These are discussed in terms of ion hopping and rotation of bound water molecules.